The invention relates generally to power generation and the efficient recovery of carbon dioxide. More particularly, the invention relates to the integration of gas-turbine exhaust compression and recirculation with carbon dioxide separation and recovery.
Power generation systems that combust fuels containing carbon, for example, fossil fuels, produce carbon dioxide (CO2) as a byproduct during combustion as carbon is converted to CO2. Carbon dioxide (CO2) emissions from power plants utilizing fossil fuels are increasingly penalized by national and international regulations, such as the Kyoto protocol, and the EU Emission Trading Scheme. With increasing cost of emitting CO2, CO2 emission reduction is important for economic power generation. Removal or recovery of the carbon dioxide (CO2) from power generation systems, such as from the exhaust of a gas turbine, is generally not economical due to the low CO2 content and low (ambient) pressure of the exhaust. Therefore, the exhaust containing the CO2 is typically released to the atmosphere, and does not get sequestered into oceans, mines, oil wells, geological saline reservoirs, and so on.
Gas turbine plants operate on the Brayton cycle. They use a compressor to compress the inlet air upstream of a combustion chamber. Then the fuel is introduced and ignited to produce a high temperature, high-pressure gas that enters and expands through the turbine section. The turbine section powers both the generator and compressor. Combustion turbines are also able to burn a wide range of liquid and gaseous fuels from crude oil to natural gas.
There are three generally recognized ways currently employed for reducing CO2 emissions from such power stations. The first method is to capture CO2 after combustion with air from the exhaust gas, wherein the CO2 produced during the combustion is removed from the exhaust gases by an absorption process, membranes, diaphragms, cryogenic processes or combinations thereof. This method, commonly referred to as post-combustion capture, usually focuses on reducing CO2 emissions from the atmospheric exhaust gas of a power station. A second method includes reducing the carbon content of the fuel. In this method, the fuel is first converted into H2 and CO2 prior to combustion. Thus, it becomes possible to capture the carbon content of the fuel before entry into the gas turbine. A third method includes an oxy-fuel process. In this method, pure oxygen is used as the oxidant as opposed to air, thereby resulting in a flue gas consisting of carbon dioxide and water.
The main disadvantage of the post-combustion CO2 capture processes is that the CO2 partial pressure is very low on account of the low CO2 concentration in the flue gas (typically 3-4% by volume for natural gas fired power plants) and therefore large and expensive devices are needed for removing the CO2. Although the CO2 concentration at the stack and thus the partial pressure could be increased by partial recirculation of the flue gas to the compressor of the gas turbine it still remains fairly low (about 6-10% by volume). The low CO2 partial pressures and large gas volumes implicit with the form of post-combustion capture leads to very high energy costs related to CO2 removal in addition to very bulky and costly equipment. Both these factors significantly increase the cost of electricity generation. Therefore there is a need for a technique that provides for economical recovery of CO2 discharged from power generation systems (for example, gas turbines) that rely on carbon-containing fuels.